Welcome to Geometry. Sum of the angles in a triangle - Geometry. Area of a Triangle - Geometry. Quadrilaterals, an overview - Geometry. Introduction to Polygons - Geometry. Introduction to angles using degrees (Geometry). Complementary, Supplementary & Vertical Angles - Geometry. Parallel Lines and Transversals - Geometry. How do you find the center of a circle? (Geometry). Introduction to the Pythagorean Theorem (Geometry). Pythagorean Theorem Proof (Geometry).